The use of HTc superconductive multifilament strands with AC causes power losses due to induced currents. It is known to reduce such losses by twisting the conductor at a very small pitch with a very small filament diameter. However, that is effective only if the filaments are electrically decoupled from one another by means of a resistive barrier.
It is known to make HTc multifilament strands by the "powder in tube" technique. That consists in filling a billet with powder reagents that are suitable, after heat treatment, for transforming into a superconductive material, and in particular into a material of the HTc ceramic type.
The billet is then closed under a vacuum and drawn down, after which it is put into a bundle in a new billet, itself in turn closed under a vacuum and then drawn down. The resulting multifilament strand may be subjected to the same steps, and so on until a desired number of filaments per unit area has been obtained.
The strand made in this way is then put into its final form, e.g. by rolling and/or twisting, and is then subjected to heat treatment to transform its powder reagents.
The material constituting the billets must be sufficiently ductile to be capable of withstanding the various drawing-down and rolling stages, and its composition must be inert or at least without consequence for the heat treatment that transforms the powder reagents into a superconductive phase. it is known that silver can be used as the material constituting the billets.
However, silver is a material that is very highly conductive at the operating temperatures of HTc superconductors. As a result there is practically no electrical decoupling between the filaments.
It is known that Ag can be doped with impurities of the Pd or Au type to 1% or 2%. That technique makes it possible to gain two decades in resistivity at 20 K.
However, the Ag/Pd alloy is expensive which makes it economically inconceivable in mass production applications.
It is also known to make at least one multilayer composite billet comprising at least one outer thickness of metal alloy, at least one of whose components is oxidizable, and an inner thickness of silver.
Heat treatment is then applied so that the oxidizable components of the metal alloy diffuse at the interface between the metal alloy and the silver and, in the presence of oxygen or of oxygen-containing compounds, oxidize to form an insulating oxide at said interface.
Filament decoupling is thus significantly improved. However, making such an oxide barrier consumes oxygen, thereby preventing the precursors from being properly synthesized into the superconductive phase.
To mitigate that problem, it has been proposed (Y. B. HUANG, R. FLUKIGER, in an article presented to SPA'97, Mar. 6-8, 1997, XU'AN, China) to use as an insulating barrier a compound that behaves in the same way as the precursors, i.e. that is permeable to oxygen while being a poor superconductor at the operating temperatures of HTc multifilament strands. Such a compound is known, e.g. Bi2202 which is not a good superconductor, or Bi2212 degenerated by Al, Mg, or Ti pollution. With Bi2212, SrAl.sub.2 O.sub.4 or CaAl.sub.2 O.sub.4 is created, for example, thereby departing from the stoichiometry necessary to have a Bi2212 superconductive phase.